Generally, the sound level of a telephone bell is variable. In high-end models, the bell is a loud-speaker fed by a square wave signal of a predetermined frequency (about 1 Khz). The loud-speaker is controlled by an amplifier, the gain of which is programmable by the user by actuating the keys of the telephone set.
In low-end telephone sets, the bell is a piezoelectric buzzer fed by the same square wave signal. Generally, the sound level of the buzzer is fixed by a potentiometer connected in series with the buzzer. In such a low-end set, the installation of a potentiometer is expensive and its use is not convenient. It would be a priori less expensive to adjust the sound level by means of the telephone keys.
However, problems are encountered in achieving a programmable gain amplifier for a piezoelectric buzzer. Indeed, a piezoelectric buzzer is a highly capacitive load liable to cause an instability of the amplifier. The amplifier should be carefully designed and compensated with relatively high value capacitors. Hence, such an amplifier would be much more expensive than the amplifier used for loud-speakers.
It is also not possible to use a pulse width modulation (PWM) amplifier. A piezoelectric buzzer exhibits several resonance peaks at various frequencies. Generally, the buzzer is fed by a rectangular signal having a 50% duty cycle and a frequency close to one of the resonance frequencies. The 50% duty cycle rectangular signal has harmonics of rapidly decreasing amplitude only slightly exciting other resonance peaks. When the duty cycle of the excitation signal is decreased, the amplitudes and the number of harmonics increase and can excite other resonance peaks, which may cause an insufficient variation of the sound level.